This application claims benefit of Japanese Application No. 2000-392116 filed in Japan on Dec. 25, 2000, the contents of which are incorporated by this reference.
1. Field of the Invention
The present invention relates to image-forming optical systems. More particularly, the present invention relates to a decentered optical system with a reflecting surface having a power for use in optical apparatus using a small-sized image pickup device, e.g. video cameras, digital still cameras, film scanners, and endoscopes.
2. Discussion of Related Art
Recently, with the achievement of small-sized image pickup devices, image-forming optical systems for use in video cameras, digital still cameras, film scanners, endoscopes, etc. have also been demanded to be reduced in size and weight and also in cost. Further, there have recently been commercially available portable telephones, PDAs and notebook personal computers that have built-in electronic image pickup optical systems. Accordingly, there are strong demands that such optical systems should be further reduced in thickness.
Under these circumstances, there have recently been proposed optical systems designed to be compact and thin by giving a power to a reflecting surface, which produces no chromatic aberration, and folding an optical path in the optical axis direction.
Japanese Patent Application Unexamined Publication Numbers (hereinafter referred to as xe2x80x9cJP(A)xe2x80x9d) Hei 8-292371, 9-90229 and 10-68884 each disclose an optical system in which an optical path is folded by a single prism or a plurality of mirrors integrated into a single block, and an image is relayed in the optical system to form a final image.
In these conventional examples, however, the number of reflections increases because the image is relayed. Accordingly, surface accuracy errors and decentration accuracy errors are transferred while being added up. Consequently, the accuracy required for each surface becomes tight, causing the cost to increase unfavorably.
JP(A) Hei 9-211331 discloses an example of an optical system in which no relay image is formed. In this example, an optical path is folded by using a single prism to achieve a reduction in size of the optical system. However, the optical system has an extremely narrow photographic field angle and is not satisfactorily corrected for aberrations.
JP(A) Hei 10-20196 discloses a zoom lens system as an example of an optical system using two prisms, in which no relay image is formed. The zoom lens system comprises two units, i.e. a positive front unit and a negative rear unit. The positive front unit includes a prism of negative power placed on the object side of a stop and a prism of positive power placed on the image side of the stop. JP(A) Hei 10-20196 also discloses an example in which the positive front unit, which comprises a prism of negative power and a prism of positive power, is divided into two to form a three-unit zoom lens system having a negative unit, a positive unit and a negative unit. In these examples, however, the zoom lens systems are not approximately telecentric on the image side and hence incapable of being applied to optical systems using image pickup devices such as CCDs. Further, because the two prisms have a total of four reflecting surfaces, manufacturing tolerances are tightened, causing costs to increase unfavorably. Further, if two reflecting surfaces are used in the prism positioned on the object side of the aperture stop, the height of extra-axial rays becomes high, causing the prism to become large in size undesirably.
JP(A) 2000-111800 also discloses lens systems each using two prisms. However, these lens systems are not satisfactorily thin and compact. Further, an image pickup device (image plane) is not placed at right angles to the photographing direction. All things considered, including the substrate of the image pickup device, it is difficult to form a very thin image pickup unit.
When a general refracting optical system is used to obtain a desired refracting power, chromatic aberration occurs at an interface surface thereof according to chromatic dispersion characteristics of an optical element. To correct the chromatic aberration and also correct other ray aberrations, the refracting optical system needs a large number of constituent elements, causing costs to increase. In addition, because the optical path extends straight along the optical axis, the entire optical system undesirably lengthens in the direction of the optical axis, resulting in an unfavorably large-sized image pickup apparatus.
In decentered optical systems such as those described above in regard to the prior art, an imaged figure or the like is undesirably distorted and the correct shape cannot be reproduced unless the formed image is favorably corrected for aberrations, particularly rotationally asymmetric distortion.
Furthermore, in a case where a reflecting surface is used in a decentered optical system, the sensitivity to decentration errors of the reflecting surface is several times as high as that in the case of a refracting surface, and as the number of reflections increases, decentration errors that are transferred while being added up increase correspondingly. Consequently, manufacturing accuracy and assembly accuracy, e.g. surface accuracy and decentration accuracy, required for reflecting surfaces become even more strict.
The present invention was made in view of the above-described problems with the prior art.
Accordingly, an object of the present invention is to provide a high-performance and low-cost image-forming optical system having a reduced number of constituent optical elements.
Another object of the present invention is to provide a high-performance image-forming optical system that is made extremely thin, particularly in a direction perpendicular to an image pickup device, by folding an optical path using only three reflecting surfaces.
To attain the above-described objects, the present invention provides an image-forming optical system having a positive refracting power as a whole for forming an object image. The image-forming optical system has a first prism and a second prism, each of which is formed from a medium having a refractive index (n) larger than 1.3 (n greater than 1.3). The image-forming optical system includes, in order from the object side, a front unit including at least the first prism, an aperture stop, and a rear unit including the second prism. The image-forming optical system does not form an intermediate image. The first prism has three optical functional surfaces transmitting or reflecting a light beam. When the three optical functional surfaces are defined as a first-first surface, a first-second surface, and a first-third surface, respectively, the first-first surface allows a light beam from the object side to enter the first prism through it. The first-second surface reflects the light beam entering through the first-first surface within the first prism. The first-third surface allows the light beam reflected from the first-second surface to exit the first prism through it. The second prism has four optical functional surfaces transmitting or reflecting a light beam. When the four optical functional surfaces are defined as a second-first surface, a second-second surface, a second-third surface, and a second-fourth surface, respectively, the second-first surface allows a light beam from the object side to enter the second prism through it. The second-second surface reflects the light beam entering through the second-first surface within the second prism. The second-third surface reflects the light beam reflected from the second-second surface within the second prism. The second-fourth surface allows the light beam reflected from the second-third surface to exit the second prism through it. The second-first surface and the second-second surface are disposed to face each other across the above-described medium. The second-third surface and the second-fourth surface are disposed to face each other across the medium. An optical path connecting the second-first surface and the second-second surface intersects an optical path connecting the second-third surface and the second-fourth surface. The first-second surface of the first prism has a rotationally asymmetric surface configuration that gives a power to a light beam and corrects aberrations due to decentration. The second-second surface and the second-third surface of the second prism have a curved surface configuration that gives a power to a light beam. The curved surface configuration is a rotationally asymmetric surface configuration that corrects aberrations due to decentration.
The reasons for adopting the above-described arrangement in the present invention, together with the function thereof, will be described below in order.
The image-forming optical system according to the present invention, which is provided to attain the above-described objects, has a first prism and a second prism, each of which is formed from a medium having a refractive index (n) larger than 1.3 (n greater than 1.3). The image-forming optical system includes, in order from the object side, a front unit including at least the first prism, an aperture stop, and a rear unit including the second prism. The image-forming optical system does not form an intermediate image.
A refracting optical element such as a lens is provided with a power by giving a curvature to an interface surface thereof. Accordingly, when rays are refracted at the interface surface of the lens, chromatic aberration unavoidably occurs according to chromatic dispersion characteristics of the refracting optical element. Consequently, the common practice is to add another refracting optical element for the purpose of correcting the chromatic aberration.
Meanwhile, a reflecting optical element such as a mirror or a prism produces no chromatic aberration in theory even when a reflecting surface thereof is provided with a power, and need not add another optical element only for the purpose of correcting chromatic aberration. Accordingly, an optical system using a reflecting optical element allows the number of constituent optical elements to be reduced from the viewpoint of chromatic aberration correction in comparison to an optical system using a refracting optical element.
At the same time, a reflecting optical system using a reflecting optical element allows the optical system itself to be compact in size in comparison to a refracting optical system because the optical path is folded in the reflecting optical system.
Reflecting surfaces require a high degree of accuracy for assembly and adjustment because they have high sensitivity to decentration errors in comparison to refracting surfaces. However, among reflecting optical elements, prisms, in which the relative positional relationship between surfaces is fixed, only need to control decentration as a single unit of prism and do not need high assembly accuracy and a large number of man-hours for adjustment as are needed for other reflecting optical elements.
Furthermore, a prism has an entrance surface and an exit surface, which are refracting surfaces, and a reflecting surface. Therefore, the degree of freedom for aberration correction is high in comparison to a mirror, which has only a reflecting surface. In particular, if the prism reflecting surface is assigned the greater part of the desired power to thereby reduce the powers of the entrance and exit surfaces, which are refracting surfaces, it is possible to reduce chromatic aberration to a very small quantity in comparison to refracting optical elements such as lenses while maintaining the degree of freedom for aberration correction at a high level in comparison to mirrors. Furthermore, the inside of a prism is filled with a transparent medium having a refractive index higher than that of air. Therefore, it is possible to obtain a longer optical path length than in the case of air. Accordingly, the use of a prism makes it possible to obtain an optical system that is thinner and more compact than those formed from lenses, mirrors and so forth, which are placed in the air.
In addition, an image-forming optical system is required to exhibit favorable image-forming performance as far as the peripheral portions of the image field, not to mention the performance required for the center of the image field. In the case of a general coaxial optical system, the sign of the ray height of extra-axial rays is inverted at a stop. Accordingly, if optical elements are not in symmetry with respect to the stop, off-axis aberrations are aggravated. For this reason, the common practice is to place refracting surfaces at respective positions facing each other across the stop, thereby obtaining a satisfactory symmetry with respect to the stop, and thus correcting off-axis aberrations.
Accordingly, the present invention adopts an arrangement in which two prisms are placed and which takes into full consideration the symmetry with respect to the stop, thereby enabling not only axial aberrations but also off-axis aberrations to be favorably corrected. In the case of an arrangement using only one prism, asymmetry with respect to the stop increases, and off-axis aberrations are aggravated unavoidably.
For the reasons stated above, the present invention adopts a basic arrangement in which the image-forming optical system has a first prism and a second prism and includes, in order from the object side, a front unit including at least the first prism, an aperture stop, and a rear unit including the second prism. Moreover, the image-forming optical system does not form an intermediate image.
It should be noted that the image-forming optical system can also be constructed in a thin structure by forming either or both of the two prisms from mirrors alone, as will be clear from Examples (described later).
Further, in the present invention, the first prism placed in the front unit has three optical functional surfaces transmitting or reflecting a light beam. When the three optical functional surfaces are defined as a first-first surface, a first-second surface, and a first-third surface, respectively, the first-first surface allows a light beam from the object side to enter the first prism through it. The first-second surface reflects the light beam entering through the first-first surface within the first prism. The first-third surface allows the light beam reflected from the first-second surface to exit the first prism through it.
If two or more reflecting surfaces are used in the prism positioned on the object side of the aperture stop, the height of extra-axial rays becomes high, causing the prism to become large in size. Therefore, the first prism used in the front unit has only one reflecting surface as stated above. Thus, it is possible to attain reductions in the size and weight of the optical system.
Further, in the present invention, the second prism placed in the rear unit has four optical functional surfaces transmitting or reflecting a light beam. When the four optical functional surfaces are defined as a second-first surface, a second-second surface, a second-third surface, and a second-fourth surface, respectively, the second-first surface allows a light beam from the object side to enter the second prism through it. The second-second surface reflects the light beam entering through the second-first surface within the second prism. The second-third surface reflects the light beam reflected from the second-second surface within the second prism. The second-fourth surface allows the light beam reflected from the second-third surface to exit the second prism through it. The second-first surface and the second-second surface are disposed to face each other across the medium. The second-third surface and the second-fourth surface are disposed to face each other across the medium. An optical path connecting the second-first surface and the second-second surface intersects an optical path connecting the second-third surface and the second-fourth surface.
If a prism in which optical paths intersect each other as stated above is used as the second prism, it is possible to construct the second prism in a compact form. The reason for this is as follows. In a comparison between the prism structure of the present invention and a prism structure of the same two-reflection type which has the same optical path length as that of the above-described prism structure and in which a Z-shaped optical path is formed, the prism structure of the present invention provides a higher space utilization efficiency. In the prism configuration having a Z-shaped optical path, rays within the prism invariably travel through different regions one by one, whereas in the prism in which the optical paths intersect each other, rays pass through the same region twice. Accordingly, the prism can be made compact in size.
Further, in the present invention, the aperture stop is positioned between the front unit including the first prism and the rear unit including the second prism, and the optical system uses only three reflecting surfaces in total. If four or more reflecting surfaces are used, the optical path of the optical system becomes complicated, and, at the same time, assembling tolerances required in the manufacturing process are tightened. This causes the production cost to increase. In contrast, the present invention uses only three reflecting surfaces in total to form the optical system. Therefore, it is possible to attain an extremely low-cost optical system and to correct decentration aberrations favorably.
Further, a combination of two prisms allows construction of an image-forming optical system that is extremely thin as a whole. Further, because an image pickup device can be disposed at right angles to the viewing direction, it becomes easy to integrate the circuit board for driving the image pickup device into an apparatus incorporating the image-forming optical system. Accordingly, the apparatus can be constructed in a slim form.
To form an image-forming optical system from a system having three reflecting surfaces in total as stated above, each reflecting surface is arranged to have a surface configuration that gives a power to a light beam. Further, to correct decentration aberrations due to the fact that each reflecting surface is decentered, each reflecting surface is arranged to have a rotationally asymmetric surface configuration that corrects decentration aberrations.
The above-described rotationally asymmetric surface used in the present invention should preferably be a plane-symmetry free-form surface having only one plane of symmetry. Free-form surfaces used in the present invention are defined, for example, by Equation (a) shown in U.S. Pat. No. 6,124,989 [JP(A) 2000-66105]. The Z-axis of the defining equation is the axis of a free-form surface.
Let us assume that the decentration direction of the image-forming optical system is a Y-axis direction; a plane parallel to the axial principal ray is a YZ-plane; and a direction perpendicularly intersecting the YZ-plane is an X-axis direction. Let us further assume that the power in the X-axis direction of the front unit including the first prism is P1x; the power in the Y-axis direction of the front unit is P1y; the power in the X-axis direction of the rear unit including the second prism is P2x; the power in the Y-axis direction of the rear unit is P2y; the power in the X-axis direction of the image-forming optical system is Px; and the power in the Y-axis direction of the image-forming optical system is Py. In the present invention, it is desirable that the front unit should satisfy the following conditions:
xe2x88x920.5 less than P1x/Px less than 0.4xe2x80x83xe2x80x83(1-1)
xe2x88x920.5 less than P1y/Py less than 0.4xe2x80x83xe2x80x83(2-1)
It should be noted that the power and focal length of the decentered optical system are defined, for example, on the basis of FIG. 15 in U.S. Pat. No. 6,124,989 [JP(A) 2000-66105].
In the conditions (1-1) and (2-1), the power of the front unit is standardized by the power of the image-forming optical system. If P1x/Px or P1y/Py is not larger than the lower limit, i.e. 0.5, the negative power of the front unit becomes very strong, and hence aberrations are aggravated. If this power is assigned mainly to the reflecting surface of the prism, because the ray height, particularly the height of extra-axial principal rays, is high, the amount of comatic decentration aberration produced by the reflecting surface increases unfavorably. If a strong power is given to a refracting surface of the prism or a lens, chromatic aberration occurs to a considerable extent at the refracting surface, causing performance degradation. The chromatic aberration cannot be corrected unless the number of constituent lens elements is increased.
If P1x/Px or P1y/Py is not smaller than the upper limit of each of the conditions (1-1) and (2-1), i.e. 0.4, the power of the front unit becomes a strong positive power, and it becomes very difficult to ensure the required back focus because the second prism, in which there are two reflections, is placed in the rear unit.
It is preferable to satisfy the following conditions:
xe2x88x920.5 less than P1x/Px less than 0xe2x80x83xe2x80x83(1-2)
xe2x88x920.5 less than P1y/Py less than 0xe2x80x83xe2x80x83(2-2)
It is desirable that the rear unit should satisfy the following conditions:
0.3 less than P2x/Px less than 2.0xe2x80x83xe2x80x83(3-1)
0.3 less than P2y/Py less than 2.0xe2x80x83xe2x80x83(4-1)
In the conditions (3-1) and (4-1), the power of the rear unit is standardized by the power of the image-forming optical system. If P2x/Px or P2y/Py is not larger than the lower limit, i.e. 0.3, the power of the rear unit becomes very weak. Therefore, it becomes impossible to attain a wide-field angle optical system. At the same time, the optical system becomes large in size unfavorably. If the power is given to the front unit, the resulting optical system is not in the form of a retrofocus type comprising a negative front unit and a positive rear unit. Consequently, it becomes extremely difficult to ensure the required back focus.
If P2x/Px or P2y/Py is not smaller than the upper limit of each of the conditions (3-1) and (4-1), i.e. 2.0, the power of the rear unit becomes very strong. Thus, the power of the reflecting surfaces becomes strong. Consequently, aberrations, particularly axial astigmatism and off-axis comatic decentration aberration, occur to a considerable extent, and it is difficult to correct these aberrations.
It is preferable to satisfy the following conditions:
0.5 less than P2x/Px less than 1.5xe2x80x83xe2x80x83(3-2)
0.5 less than P2y/Py less than 1.5xe2x80x83xe2x80x83(4-2)
Further, it is desirable to satisfy the following condition:
70xc2x0 less than xcex8 less than 110xc2x0xe2x80x83xe2x80x83(5-1)
where xcex8 is the angle formed between the axial principal ray incident on the first-second surface and the axial principal ray reflected therefrom.
The condition (5-1) specifies the angle formed between the axial principal ray incident on the reflecting surface of the first prism and the axial principal ray reflected from the reflecting surface. If the angle xcex8 is not smaller than the upper limit of the condition (5-1), i.e. 110xc2x0, or not larger than the lower limit, i.e. 70xc2x0, the angle at which the axial principal ray enters the second prism deviates largely from an angle at which the axial principal ray is substantially parallel to the image plane (i.e. the axial principal ray is undesirably tilted). Consequently, it becomes impossible to attain a reduction in size of the optical system. In particular, if the angle xcex8 is not larger than the lower limit, the second prism undesirably projects toward the object side to a considerable extent. If the angle xcex8 is not smaller than the upper limit, the image plane has to be tilted to a considerable degree. Alternatively, the optical system has to be arranged so that an extra-axial light beam is incident obliquely on the image plane. Consequently, it becomes impossible to attain a telecentric optical system.
It is preferable to satisfy the following condition:
75xc2x0 less than xcex8 less than 100xc2x0xe2x80x83xe2x80x83(5-2)
Further, it is desirable to satisfy the following condition:
0.5 less than D/f less than 2.5xe2x80x83xe2x80x83(6-1)
where D is the thickness of the image-forming optical system, and f is the mean focal length of the image-forming optical system.
It should be noted that the term xe2x80x9cthickness D of the image-forming optical systemxe2x80x9d means the thickness of the image-forming optical system measured from the image pickup device in a direction perpendicular to the image pickup device. As shown in FIG. 10, which is a ray path diagram illustrating Example 1 (described later), the thickness D is defined as the distance from the forwardmost ray position A among the rays within the image-forming optical system to the image plane 3. The mean focal length f is defined as f=(fx+fy)/2, where fx is the focal length in the X-axis direction of the image-forming optical system, and fy is the focal length in the Y-axis direction of the image-forming optical system.
In the condition (6-1), the thickness of the image-forming optical system is standardized by the focal length of the image-forming optical system. In the case of a decentered optical system, the optical path can be folded. Further, when the optical system is formed from a first prism and a second prism that are arranged as in the present invention, the optical system can be constructed in a very thin structure within the above-described range. If D/f is not larger than the lower limit of the condition (6-1), i.e. 0.5, it becomes impossible to ensure the required effective surface area and edge wall thickness of each prism. In particular, if the edge wall thickness is not sufficiently ensured, assembleability is degraded, and costs increase unfavorably. If D/f is not smaller than the upper limit of the condition (6-1), i.e. 2.5, the thickness of the optical system increases unfavorably. Therefore, there is no merit in using a decentered optical system to form the image-forming optical system.
It is preferable to satisfy the following condition:
0.8 less than D/f less than 2.3xe2x80x83xe2x80x83(6-2)
Further, it is desirable to satisfy the following condition:
0.1 less than SD1/f less than 3.0xe2x80x83xe2x80x83(7-1)
where SD1 is the optical path length of the axial principal ray from the first-second surface of the front unit, which is a reflecting surface, to the aperture stop.
In the condition (7-1), the optical path length of the axial principal ray from the first reflecting surface of the front unit to the aperture stop is standardized by the focal length of the image-forming optical system. If SD1/f is not larger than the lower limit of the condition (7-1), i.e. 0.1, it becomes necessary to increase the power of the reflecting surface in order to ensure the desired back focus. As a result, off-axis comatic decentration aberration increases unfavorably. If SD1/f is not smaller than the upper limit of the condition (7-1), i.e. 3.0, the size of the apparatus increases unfavorably although it is possible to ensure the desired back focus without the need to increase the power of the reflecting surface.
It is preferable to satisfy the following condition:
0.2 less than SD1/f less than 2.0xe2x80x83xe2x80x83(7-2)
Further, it is desirable to satisfy either of the following conditions:
0.5 less than SD2/f less than 3.0xe2x80x83xe2x80x83(8-1)
2.0 less than SD3/f less than 6.0xe2x80x83xe2x80x83(9-1)
where SD2 is the optical path length of the axial principal ray from the first surface of the front unit, which is closest to the object side, to the aperture stop, and SD3 is the optical path length of the axial principal ray from the aperture stop to the final surface of the rear unit that is closest to the image side.
It should be noted that the above-described first surface and final surface do not include the surface of a plane-parallel plate that may be provided on the object or image side of the optical system.
In the condition (8-1), the optical path length of the axial principal ray from the first surface of the front unit to the aperture stop is standardized by the focal length of the image-forming optical system. In the condition (9-1), the optical path length of the axial principal ray from the aperture stop to the final surface of the rear unit is standardized by the focal length of the image-forming optical system.
If SD2/f is not larger than the lower limit of the condition (8-1), i.e. 0.5, the optical path length of the axial principal ray becomes excessively short, and it becomes difficult to ensure the required back focus. If SD2/f is not smaller than the upper limit of the condition (8-1), i.e. 3.0, the first prism becomes large in size. As a result, the overall size of the lens system increases unfavorably.
If SD3/f is not larger than the lower limit of the condition (9-1), i.e. 2.0, it becomes difficult to correct decentration aberrations although it is advantageous to ensuring the required back focus. If SD3/f is not smaller than the upper limit of the condition (9-1), i.e. 6.0, the second prism becomes large in size unfavorably, and it becomes impossible to ensure the required back focus.
Regarding the condition (9-1), which is concerned with the optical path length from the aperture stop to the final surface of the rear unit, the optical path length specified by the condition (9-1) is the distance to the image plane in a case where the rear unit is formed from mirrors as in Examples 7 and 8 (described later).
It is preferable to satisfy either of the following conditions:
0.8 less than SD2/f less than 2.5xe2x80x83xe2x80x83(8-2)
2.0 less than SD3/f less than 5.0xe2x80x83xe2x80x83(9-2)
A finder optical system can be formed by using any of the above-described image-forming optical systems according to the present invention as a finder objective optical system and adding an image-erecting optical system for erecting an object image formed by the finder objective optical system and an ocular optical system.
In addition, an electronic camera apparatus can be constructed by using any of the foregoing image-forming optical systems according to the present invention, an image pickup device placed in an image plane formed by the image-forming optical system, a recording medium for recording image information received by the image pickup device, and an image display device that receives image information from the recording medium or the image pickup device to form an image for observation.
In addition, an endoscope system can be constructed by using an observation system having any of the foregoing image-forming optical systems according to the present invention and an image transmitting member for transmitting an image formed by the image-forming optical system along a longitudinal axis, and an illumination system having an illuminating light source and an illuminating light transmitting member for transmitting illuminating light from the illuminating light source along the longitudinal axis.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.